Switching system



July 28, 1942. A. J. BUSCH 2,291,024

SWITCHING SYSTEM Filed Dec. 12, 1940 2 Sheets-Sheet l '3 L 2 S u 0. G. SENDEIP lNVENTOR A J BUSCH A 7'7'ORNEV Filed Dec. 12, 1940 2 Sheets-Sheet 2- INVENTO R A. J BUSCH ATTORNEY Patente d July 28, 1942 an on SVJITCHIING SYSTEM Appiication December 12, 1949, Serial No. 369,765

8 Claims.

This invention relates to automatic switching telephone systems and particularly systems of this character having selectable devices for controlling switching connections.

The object of the invention is to provide an improved and economically arranged link switching system for automatically establishing switching connections for telephone calls in a minimum time period.

Heretofore link switching systems have been provided for selecting senders and other devices which are automatically responsive to a signal transmitted by an incoming or an outgoing trunk circuit. Systems of this kind have been shown in the patent application of G. V. King, J. B. McKim and O. Myers filed September 15, 1939, Serial No. 295,010, issued as patent 2,236,246, March 25, 1941, and patent application of A. J. Busch and F. J. Scudder filed September 15, 1939, Serial No. 295,012. Such systems provide senders or other devices available to a link switching system for selection by said link switching system when found idle and for connection to a trunk circuit through the contacts of a link switch. These systems, however, make a test of the available senders in a group and if all of the senders are busy, the link switching system is disconnected from the trunk circuit.

A feature of the present invention is a link switching system having devices for simultaneously connecting the senders of a first group of senders with sender testing facilities in search of an idle sender and when the senders tested in this group are found busy, said testing facilities are successively switched to the senders in a plu- 1 rality of diiferent groups under the control of sender testing facility switching devices and a timing device; the senders of each group being simultaneously tested by said testing facilities during the progression of said testing facilities from one group of senders to another.

Another and related feature resides in arresting the operation of the testing facility switching devices when an idle sender is selected from any of said plurality of groups.

Another and related feature resides in a link switching system according to the first feature comprising a plurality of link switches and a plurality of link control circuits each having said sender testing facilities switching devices and a timing device therein and each having a plurality of different groups of senders available thereto for test, with means for selecting an idle link control circuit responsive to the actuation cuit having means to make said successive test of diiferent sender groups and disconnect itself from said trunk after a plurality of groups of senders have been repeatedly tested for a predetermined time period and no idle sender has been found, and for arranging the link switching system so that another idle link control circuit may be selected for testing another plurality of groups of senders to find an idle sender.

Another and related feature feature resides in a link switching system according to the first feature comprising a plurality of link control circuits each having available thereto diiferent characters of senders for selection; a plurality of groups of each different character of senders being available to each control circuit and each control circuit having means therein responsive to a signal from a trunk circuit for determining the character of sender groups from which an idle sender shall be selected.

These and other features will be discussed more fully in the following description.

To illustrate the features of the invention reference may be had to the accompanying drawings in which Fig. 1 and Fig. 2 illustrate a trunk circuit diagrammatically and a link switching system comprising link switches, selectable link control circuits, a link connector, link frame relays and sender groups.

General description The general organization of circuits and equipment for use with a link switching system according to the present invention may be of the character fully disclosed in patent application Serial No. 295,012, filed by A. J'. Busch and F. J. Scudder September 15, 1939, and may be cooperatively associated with a cross-bar switching system of the character disclosed in patent application Serial No. 295,010, filed by G. V. King, J. B. McKim and O. Myers, September 15, 1939 issued as Patent 2,236,246 March 25, 1941.

In the foregoing systems a call from a toll office energizes the apparatus of an incoming intertoll trunk circuit which causes the selection of a link connector and link control circuit. The link control circuit selects an idle incoming sender and actuates a primary link switch and a secondary lmk switch for associating the incoming sender so selected with the calling incoming trunk through the contacts of these primary and secondary link switches. The incoming sender is set by impulses transmitted from the distant toll ofiice. These impulses maybe of a trunk start circuit, said selected control cir- 55 used for setting the code registers of the incoming sender and where necessary for setting both the code and numerical registers of this sender. The energization of the incoming sender associates a marker therewith and causes the transfer of the code registers from the sender to this marker. In response to the energization of the code registering equipment in the marker other equipment is operated for causing the selection of an idle outgoing trunk to the desired distant oflice. The marker selects an idle outgoing trunk and causes the energization of switches for connecting the incoming and outgoing trunks together. For calls requiring the transmission of numerical digits an outgoing sender is selected by the outgoing trunk circuit after a signal is transmitted to the outgoing trunk for this selection. To obtain the services of an outgoing sender the outgoing trunk also energizes a link connector and link control circuit causing the selection of an idle outgoing sender from a large plurality of senders, which, after it is selected, is connected through primary and secondary switches of the link circuit to the outgoing trunk. The numerical digits are transferred from the incoming sender through the contacts of the secondary and primary link switches first selected by the incoming trunk, through the incoming trunk circuit, thence over cross-bar switches to the outgoing trunk circuit, thence over the contacts of the primary and secondary link switches selected by the outgoing trunk, to the registers of the selected outgoing sender. The numerical registrations received by the outgoing sender are transmitted to a distant oflice over the contacts of the secondary and primary switches associated with the outgoing sender and over the outgoing trunk conductors to the distant ofiice.

The trunk circuit I diagrammatically illustrated in Fig. 1 of the present embodiment may be considered as either an incoming or outgoing trunk of the same character as the intertoll trunk illustrated inFigs. 1 and '7 of the aforementioned patent application filed by A. J. Busch and F. J.

Scudder. This trunk may be used either as a trunk incoming from the distant oifice or an outgoing trunk for connecting calls from this office to a distant office. The energization of this trunk when used as an incoming trunk establishes a start circuit which governs the selection of an idle link connector and a link control circuit for associating an incoming sender and marker with the incoming trunk.

The type of equipment to be selected is governed bythe intertoll trunk circuit. For example, when the two-way intertoll trunk circuit is used as an incoming intertoll trunk, a class signal is transmitted to the control circuit indicating that an idle incoming sender is required. The control circuit is then associated with a plurality of incoming senders which are also connected to certain levels of the secondary crossbar link switches. Each control circuit may be connected to a different plurality of senders, some of which are the same as those connected to other control circuits. Assuming that there are one hundred and fifty incoming senders and ten control circuits, the one hundred and fifty senders are divided among the control circuits in any manner desired. An example may be assumed in which ten senders may be simultaneously tested by the control circuit, which ten senders are connected to ten different levels of a plurality of secondary link switches. An arrangement is provided, however, to prevent the test of senders which are connected to busy secondary link switches. This busy condition is indicated by a ground connected to the hold magnet of link switches associated with busy senders. When an idle sender is chosen from a group of senders the select magnet of the proper link switch level is operated and the hold magnets of the proper verticals in both the primary link switch and the secondary link switch are operated. The incoming intertoll trunk is thus connected through the primary and secondary link switches to the idle incoming sender selected. A signal is transmitted to the distant calling operator that pulses may now be transmitted for setting the registers of the sender. During this period the apparatus of the incoming sender is energized for selecting an idle marker. This is accomplished through the energization of a marker connector in the manner fully described in Patent 2,236,246 to King-McKim-Myers, filed September 15, 1939 and'is'sued March 25, 1941.

When the telephone connection requires the transmission of numerical pulses the outgoing trunk energizes a link start circuit for establishing connection with a link switching system for selecting a link control circuit for obtaining the services of an outgoing sender. When the link control circuit is associated with the outgoing trunk a distinctive class signal circuit'is established from the outgoing trunk circuit to the link control circuit which causes the link control circuit to test a plurality of outgoing senders in order to find an idle sender. The outgoing senders testedare associated with a particular plurality of levels of the secondary link switches. When an idle out-going sender is found, the proper select magnets and hold magnets of the primary link switch and the secondary link switch are operated. This associates an outgoing sender with the outgoing trunk over link switch contacts and since the incoming trunk is connected to an incoming sender over other link switch contacts, circuits may be completed at this time for connecting the incoming sender to the outgoing sender through the incoming trunk, cross-bar switches and an outgoing trunk. A signal is given from the outgoing sender to the incoming sender and the numerical pulses are transmitted from the incoming sender for setting the registers of the outgoing sender. This ordinarily completes the services of the incoming sender and at this time this sender may be released, which releases the hold magnets of the link switches which associated the incoming sender with the incoming trunk.

The present invention contemplates the foregoing arrangement of link switches and in addition contemplates control circuit devices forselecting an idle sender in a minimum time period for telephone connections during heavy load intervals by successively and repeatedly connecting link control circuit testing facilities with a plurality of different groups of senders under the control of testing facility switching devices and a timing device when all of the individual senders in the groups tested are busy. A plurality of different groups of senders are thus made available to the link control circuit testing facilities each group having a large plurality of senders therein. The senders of each group are simultaneously tested and consequently the rapid transfer of testing facilities in the control circuit successively and repeatedly from one group to another to find an idle sender has the advantage of satisfying the requirements forcompleting a telephone call in the shortest possible time. The sender in the different groups may be cross-connected in a, manher to be available to the testing facilities of a plurality of link control circuits and when so connected are arranged in a difierent order with relation to the testing facilities of the different control circuits so that the first order of senders tested by one control circuit may be the second or third order of senders tested by another control circuit. This makes available a large plurality of senders to the testing facilities of each control circuit with a minimum amount of equipment.

The testing facilities of the link control circuit are first connected to the senders of one group and if an idle sender is found in this group it is taken for use and the control circuit testing facilities are not associated with another group of senders. When the senders of the first group are all found busy, the link control circuit testing facilities are automatically transferred to the senders of another group which are simultaneously tested by the testing facilities in an effort to find an available idle sender. When the senders of the latter group are all found busy a transfer is made to another group of senders or to the first group of senders tested to repeat the test of these senders in an effort to find an idle sender which may have been released by another telephone connection. This automatic transfer of the testing facilities continues from one group of senders to another for a period of time until an idle sender is found or this period of time expires. The action of the testing facility switching devices is automatically arrested when an idle sender is found in any sender group. If no idle sender is found in the allotted period of time the link control circuit is disconnected from the trunk circuit and the trunk is automatically associated with another control circuit which makes a progressive test of the groups of senders associated therewith. This continues as long as a sender is demanded by a trunk circuit.

Fundamentally, the arrangement is as follows: Circuits are established by trunk circuit equipment for selecting a link control circuit and for operating a class relay combination for directing the control circuit to select a particular character of equipment. In this instance, groups of senders have been illustrated as selectable equipment. Two groups of incoming senders and two groups of outgoing senders are shown, each group having a plurality of senders therein. The class signal, therefore, directs the control circuit to select a sender either from a group of incoming senders or a sender from a group of outgoing senders. Before the class signal may be effective a control circuit is selected and associated with the trunk circuit. A multicontact relay and a connector are used for associating the selecting control circuit with the trunk circuit and with relays which are selectively operated for associating a particular group of senders with the control circuit. After the class relay combination is energized, other equipment in the control circuit associates testing facilities with a group of senders of the character directed by the trunk circuit for simultaneously testing the senders in this group. The testing facilities are shown in the right portion of Fig. 1 and constitute relays I3II to I31 which are associated with the senders of a group and with the link switches which are connected to the same senders. Relays I38, I39, I49 and I4! cooperate with relays I33 to I31, certain of which are energized when an idle sender is selected. Relays, such as 130, I3I, I32 and I 3.3, are associated with different link switch combinations, each combination comprising a vertical on a primary link switch and a vertical on a secondary link switch, each vertical being associated with a plurality of horizontal levels on these switches. The hold magnet of one vertical of the primary link switch and the hold magnet of one vertical on a secondary link switch are connected in multiple and may be known as a switch combination. These hold magnets are simultaneously energized when a sender connected to a horizontal level of the switch is selected. Ground is associated with the hold magnet for its operation and therefore when any of the relays I30, I3I, I32 and I33 are associated with a busy switch combination, the relay is operated. This disassociates the windings of testing facility relays I34, I35, I36 or I3'I from the senders connected to the busy link switch combination. There are as many relays, such as I34I35, as there are senders in an outgoing sender group and as many relays, such as I36I 37, as senders in an incoming. sender group. Relays I34 and I35, therefore, represent a plurality of relays which may be associated with senders of one group of a particular character. Relays I33 and I3! represent another plurality of relays which may be associated with a sender group having senders of a different character. The class relays 292 and 203 determine which of the relays shall be associated with the groups of senders to be tested.

Upon the actuation of the control circuit the testing facilities are chosen and associated with the senders of a group. A testing facility switching combination is also energized which allows th testing facilities such as relays I32, I33, I 33 and I3! to be associated with th senders of this group for a limited period of time which is of sufficient duration to determine whether there is an idle sender in the group. These testing facility relays I32, I33, I33 and I3? are then automatically transferred to the senders of another group of the same character if an idle sender was not found in the first group tested. When no idle sender is found in the second group tested the sender facility testing equipment is either transferred to another group of senders of the same character or returned to the first group of senders to repeat the test of senders in this group. Betwen the original test of th first group of senders and the second test of the senders in this group, it is probable that one of the senders has been released by a telephone connection which has been completed. However, if no idle sender is found the testing facility switching arrangement continues to function for transferring the testing facilities from one group of sender to another for a time period controlled by timing devices comprising interrupter 225, relay 2M and relays ZiS to 2IS, inclusive. The latter relays are energized under the control of interrupter 225 to determine the period of time for switching the link control testing facilities for successively testing the senders of a plurality of sender groups and repeating the test of senders in these groups if necessary. I

When during the foregoing regulated tim period no idle sender is found, a second function of the time circuit is to cause the release of the link control circuit. This, however, does not release the entire link switchin system." The link control circuit is restored to normal and in so doing disconnects'the trunk circuit from this link control circuit but arranges the equipment of the link switching system so that another link control circuit is immediately selected. The second link control circuit functions in the same manner since it has the same testing facilities and the same testing facility switching equipment and timing circiut. The second control circuit, however, has available different groups of senders than are available to the first link a:

control circuit selected.

When an idle sender is found, ground is associated with a lead extending to the link control circuit and operates a testing facility relay such as I34, I35, I36 or I31. This relay energizes other relays in the control circuit for arresting the operation of the testing facility switching mechanism and for associating ground with the hold magnet of the link switch vertical with which the idle sender is connected, as shown in the aforementioned patent application of A. J. Busch and F. J. Scudder. Circuits are established when an idle sender is selected to indicate to the control circuit with which horizontal level of the link switches the idle sender is connected.

The control circuit has equipment therein for energizing the select magnets associated with these levels. This equipment may be the same as in the aforementioned patent application and is not shown herein. In this manner a particular idle sender is chosen from a large plurality of senders and th associated link switch is energized by the control circuit for connecting the selected idle sender with the trunk circuit which caused the selection of the control circuit.

Detailed description A trunk circuit which requires the services of a link and link control circuit either in response to an incoming call or in response to an outgoing connection energizes a link start circuit and a class indicating circuit by the operation of relays in the trunk circuit. The start circuit for engaging the services of a link and link control circuit may be traced from ground I02 in the trunk Each control circuit has a representative relay in each connector so that any one of the control circuits may be associated when idle with any of a large plurality of trunks in any of a plurality of trunk groups. The sequence circuit and connector are energized by battery through resistanc I5I, contact 4 of relay I21 to the sequence circuit relays of connector I29. Ground is extended from contact 2 of relay I21 through the sequence circuit shown in dotted lines, winding of relay I28 to battery operating the multicontact relay I28. Assuming that the control circuit shown in Fig. 2 and the right portion of Fig. 1 is the idle control circuit selected, a circuit is immediately established for energizing the offnormal relay 2I5 from battery, through its windlng, contact I of relay 222 to ground on contact I of relay I 28. A circuit is also established for a trunk group relay such as relay 226 from ground on contact I of relay I21, through contact I2 of relay I28, winding of relay 226 to battery. Each start relay, such as I21, is associated with a particular group of trunks. The control circuit is equipped with trunk group relays such as relay 226, equivalent in number to the number of trunk groups served by a link group combination. For example, if there are ten trunks in a group and the link is arranged to serve ten such groups of trunks, there would be one hundred trunks in the group which would be represented by ten trunk group relays 226 in the control circuit. The operation of the off-normal relay 2I5 operates the associated trunk group relay I26 in the link frame circuit through the contact of a particular trunk group relay 226 in the link control circuit. The operation of relay I26 associates the control circuit with a plurality of sender group relays and also establishes a local circuit for the operation of relay I50. This circuit may be traced from ground, through contact 5 of relay I26, contact I5 of relay I28, winding of relay I50 to battery. A locking circuit is also established through contact 5 of relay I26 for the start relay I21. Relay I50 opens the original energizing circuit for the start relay I21 and this relay remains operated over its locking circuit until the release of relay I26.

The trunk circuit I00 indicates to the sender whether it requires the services of an incoming sender or an outgoing sender or other devices which may be selected by a control circuit of this character. A trunk circuit requiring the services of an incoming sender operates a trunk circuit relay to establish a circuit from ground through resistance I03, contact I4 of relay I28, winding of relay 203 to battery operating relay 203. A trunk circuit requiring the services of an outgoing sender operates a trunk circuit relay for establishing a circuit from ground through resistance IOI, contact I3 of relay I28, winding of relay 202 operating the latter relay. These are known as class relays to indicate to the link control circuit the character of equipment to be selected. Assuming the services of an incoming sender are required, relay 203 is energized which establishes a circuit from battery, through resistance 228, contact I of relay 204, contact 2 of relay 203, contact 2 of relay 202 to the contacts of transfer relays 200 and 20I. This circuit is prepared in advance of the operation of either relay 200 or 20I. Relays 201 and 209 control the successive operation of relays 200 and 20I.

Selecting an idle sender in first group tested A number of relays are simultaneously energized at the time that the control circuit offnormal relay 2I5 is operatedwhich immediately associates the testing facilities of the control circuit with a group of senders, and initiates the control of switching relays and a timing circuit. The timing device is energized by the operation of relay 2| 4 from battery, through its winding, contact 2 of relay 2I5 to ground on contact 2 of relay 222. The operation of this timing device will be described later. Relay 2I0 is energized over a circuit from ground on contact 3 of relay 2I5, contact I of relay 2 to relay 2I0 to battery. The latter relay is a slow-release relay which functions in combination with relay 2I I for transferring the testing facilities from one group of senders to another when all senders in a tested group are found busy. The operation of relay 2I0 connects ground through its contact I, continuity contact of relay 209, winding of relay 209, resistance 25I to battery energizing relay 209. The energization of this relay inherently assumes that relay 261 is normal and consequently a circult is established for relay 200. This circuit may be traced from battery, through resistance 2H8,

contact I of relay ZIII, winding of relay 2%, contact 3 of relay 2H1, contact I of relay 205 to ground through contact 3 of relay 29L 'The opere ation of relay 20B establishes a circuit for connecting the senders of a group with the control circuit testing facilities shown in the right portion of Fig. 1 by energizing one of the relays I22 or I23 depending upon the character of sender wanted for the telephone connection. Since in the example being given relay ZIlIl and class relay 203 are energized, a circuit for relay I22 is established over a circuit traced from battery, through resistance 228, contact I of relay Eta, contact 2 of class relay 293 operated, contact 2 of class relay 2&2 normal, contact I of relay 259, conductor 234, contact of relay I28, contact 5 of relay I 26, winding of relay I22 to ground, energizing the latter relay. Relay I22 establishes a circuit for operating relay 285 for initiating the operation of the testing facility'switching mechanism as explained later. len incoming senders have been shown as sender group III: which are associated over ten leads IM--I I5 through ten contacts of relay I22 shown as contacts I and 2, thence over leads I69 and I51, through the contacts I and B of relay I23, connector I28, contact 2 of relay I33, winding of relay It? to battery, and contact 2 of relay I32, winding of relay I36 to battery. Assuming that there are, ten senders in a group, there would also be ten relays such as I36 and I37. When a sender is idle, ground is associated with the leads traced. Assuming that the tenth sender is idle, ground would therefore be associated from the sender through contact 2 of relay I22, lead IEI, contact 8 of relay I 28, contact 2 of relay I32, winding of relay I36 to battery which would operate relay I36. This relay establishes a locking circuit for itself through its contact 2 to the operating ground in the sender. The leads from the sender are carried through the contacts of relays such as I32 and I 33 to prevent a test of senders which are connected with busy link switches. A busy link switch has ground connected with the hold magnet for operating this magnet. This ground establishes a circuit for the relays such as I32 opening the leads extending to relays such as I36.

It has been assumed that an idle sender has been found in the first group of senders tested which causes the operation of relay I36. This relay establishes a circuit for relay I lIl from round connected to the armature and contact I. of relay I36, winding of relay It to battery. Relay I IIJ opens the circuit for relay 2| I to arrest the action of the testing facilities switching devices as described later. Ground is also connected through contact 3 of relay I 36, winding of relay Isl to battery to operate the latter relay. The operation of relay I II establishes an obvious circuit for operating relay I 38 and also establishes obvious circuits for operating relays I35, I3I, I32 and I33. The operation of the latground on contact I of relay I37, over a chain circuitthrough intermediate relays, contact I of relay I36, contact 3 of relay I38, winding of hold magnet III'S to battery which operates this hold magnet to lock cross-bar switch contacts of a horizontal level connected to the selected sender. If, however, relay I37 had been operated indicating'that the first sender is idle, there could have been other relays such as I31 and I36 associated with idle senders which would also have operated and established their locking circuits. The first idle sender, however, is the one selected since a chain circuit prevents the selection of more than one sender. This is clear from observing that ground connected with the armature I of relay I3! is disassociated from all other ter relays open all testing leads to the senders relays when relay I3'I is operated and is extended through contact l of relay I38 for energizing the link switch hold magnet I05. Since this ground is disassociated from other relays none of the other link switch hold magnets may be operated. The operation of relay I40 prevents the control circuit from making further tests as will be later described. As explained in the aforementioned patent application to A. J. Busch and F. J. Scudder after a sender is seized a second test of this sender is made to satisfy the control circuit that it is'free for use. This testing circuit is diagrammatically shown as test circuit 206 which may operate in the same manner as described in the above application. When the test is satisfactory, a circuit is established through the test relay contact for energizing test cut-01f relay 264 which operates relay 2 I2 from ground through its contact 5, winding of relay 2 I 2 to battery. Relay 2I2' energizes relay 222' for causing the release of the control circuit. The operation of relay 222 opens the circuit for the timing relay 2 I I and the ofi-normal relay 2I5 causing their release. The release of relay 2M causes the timing relays 2I6, 2H, 2I8 and ZIII to restore to normal. The release of relay 2I5 opens the circuit for the progressive testing relay m through its contact 3 and through its'contact I, opens the circuit for relay I26 causing the release of the latter relay. The. release of relay I 26 opens the holding circuit for start relay I21 which causes the release of trunk group relay 226 and the release of link relay IZS. Relay IE9 is slow in releasing so that the trunk start circuit is opened to insure the release of relay I28.

Progressive and repeated test of sender groups The foregoing gives the circuits established for testing one group of senders when an idle sender is found in this group. In presenting the arrangements of the invention, four groups of senders have been shown, two groups of incoming senders I IIl'andI I2 and two groups of outgoing senders I II and H3. Relay I22 is operated for associating the testing facilities including relays IBt-IBI with the first group of incoming senders and relay IE4 is operated for associating these testing facilities of the control circuit with the second group of incoming senders. Relay I23 is operated for associating the testing facilities of the link control circuit including relays ltd-I35 with the first group of outgoing senders and relay IE5 is operated for associating these testing facilities with the second group of outgoing senders. When all of the senders of a group are found busy, the switching equipment of the link control circuit operates in the following manner: Relays 2t! and 269 successively energize relays 203 and 2M for switching from one group of senders to another. Relays 2 I and 2 control the operation of relays 201 and 209. Relays 2I0 and 2 are progressively energized under the control of the testing facilities as they are associated with the sender groups. As previously described relay 2I0 is energized from ground on contact 3 of the off-normal relay 2I5 and in turn energizes relay 209 which looks to ground on its continuity contact. Relay 201 is normal since a ground shunt is connected to the lower terminal of its winding through contact I of relay 2I0. The senders of the sender group IIO are tested as previously described. The operation of relay I 22 for testing the senders of sender group IIO associates ground the winding of relay 205 operating the latter relay. A circuit is thus established for relay 2 from battery, through its winding, contact 2 of relay 204, contact 3 of relay 205, contact I of relay I40, contact I of relay I39 to ground. Relay 2| I is a slow operating relay and does not have time to operate if an idle sender exists in sender group IIO because when an idle sender is found in this group, a circuit is established for relay I40 which opens the circuit for relay 2I I. Relay I39 is associated with the outgoing sender testing facilities for this same purpose. neither relays I 40 nor I39 are energized and relay 2 thus has time to energize, a transfer from one group of senders to another is made in the following manner: The energization of relay 2II opens a circuit for relay 2I0 causing its release. The release of relay 2I0 removes the shunt from the winding of relay 201 and thus causes the operation of relay 201 from battery through resistance 250, winding of relay 201, contact of relay 209 to ground. The energization of relay 201 opens the circuit for relay 200 and prepares a circuit for relay I from battery through resistance 208, contact 2 of relay 201, winding of relay 20I to contact 2 of relay 2I0. Relay 2I0 must, however, again operate before this circuit is completed. The release of relay 200 opens the energizing circuit for relay I22 causing its release. The release of relay I22 opens the energizing circuit for relay 205 which causes its release. The release of relay 205 opens the energizing circuit for relay 2I I and the release of relay 2I I energizes relay 2I0. Relay 2I0 now completes the circuit for relay 20I through its contact 2, contact 2 of relay 205 to ground on contact 3 of relay 200 which was released. At this time relay 2I0 also establishes a circuit for causing the release of relay 209 by shunt. Ground from contact I of relay 2 I0 is connected through the inner contact 3 of relay 201 to a point between resistance 25I and the winding of relay 209. Since this places ground on both terminals of the winding of relay 209, the relay releases and relay 201 remains operated through the continuity contact of relay 209, winding of relay 201, resistance 250 to battery.

With relays 201 and 20I operated a circuit is established to operate the link frame relay I24 for associating the control circuit testing faciliities with a second group of incoming senders. A circuit for relay I24 may be traced from ground, through its winding, contact 2 of relay I26, contact 3 of relay I28, conductor 232, contact I of relay 20I, contact 2 of relay 202, contact 2 of relay 203, contact I of relay 204, resistance 228 to battery. It will be remembered that class relay 203 was operated for associating incoming senders with the control circuit and 202 is normal. The operation of relay I24 Assuming that.

connects ten test leads designated II8 to H9 from the senders in group II2 through contacts I and 2 of relay I24 which represent ten contacts thence through contacts 1 and 8 of relay I28,

* connector I29, contact 2 of relay I33, winding of relay I31 to battery and contact 2 of relay I32, winding of relay I36 to battery. As will be remembered ten relays such as I36 and I31 are used for testing ten senders in a group. If the first sender of a group is idle and thus has ground associated with conductor II8 relay I31 is operated and the search for an idle sender is discontinued by the operation of relays I40, HI and I38 as previously described. If, however, all of the senders in group I I2 are busy, none of the ten relays I36 to I31 will be operated and the testing of senders in other groups will continue The operation of relay I24 for associating the testing facilities of the link control circuit with the sender group. II2 also associated ground through its contact 3, contact II of relay I28, through connector I29, conductor 236, winding of relay 205 to battery for operating relay 205. Whenever the testing facilities of the control circuit are associated with the senders of any group relay 205 is operated. The operation of relay 205 initiates the transfer of the testing facilities to another group by energizing relay 2 over a circuit which may be traced from battery, through its winding, contact 2 of relay 204, contact 3 of relay 205, contact I of relay I40, contact I of relay I39 to ground. As previously shown, relay I40 and, as will be shown, relay I39 are normal when all of the senders of a group are busy and are energized immediately when an idle sender is found in a group. The operation of relay 2I I again releases relay 2I0 which causes the release of relay 201 since relay 209 was previously released. The release of relay 201 opens the circuit for relay 201 which was locked to ground through its contact 3. Since the circuit for relay 200 is now open at contact 3 of relay- 2I0 a test of another group of senders cannot be made until relay 2I0 again operates. Upon the release of relay 20I, the following takes place: Relay 20I opens the energizing circuit for relay I24; the release of relay I24 opens the energizing circuit for relay 205; the release of relay 205 opens the energizing circuit for relay 2 and the release of relay 2I I again establishes the energizing circuit for relay 2I0. With relay 2I0 energized, a circuit is again established for relay 200 or a similar relay if more than two groups of senders are provided. This circuit may be traced from battery, through resistance 208, contact I of relay 201, winding of relay 200, contact 3 of relay 2I0, contact I of relay 205, contact 3 of relay 20I to ground. The operation of relay 200 again energizes relay I22 as previously traced from ground, through its winding, contact 4 of relay I26, contact 5 of relay I28, through connector I29, conductor 234, contact I of relay 200, contact 2 of relay 202, contact 2 of relay 203, contact I of relay 204, resistance 228 to battery. The testing facilities of the link control circuit including the ten relays I36 to I 31 are again associated with the ten senders in sender group IIO. Between the time that the senders of this group were originally tested and the second test after the senders of group II2 were tested, it is usual that a sender will have been released in group I I0 and consequently an idle sender will be found. The link control circuit switching apparatus continues to function so that if an idle sender is not found in the sender group IIO, the

testing facilities will again be switched to the sender group H2. For this purpose the operation of relay 21!! again energizes relay 289 from ground through contact of relay 2m, continuity contact of relay 2G9, winding of relay 259, resistance 25! to battery. This prepares a circuit for the operation of relay 207 when relay 2H3 again is released.

Timing devices The transfer of the testing facilities of the link control circuit continues as described above for a period of time which is regulated by the timing relays and interrupter shown in the lower portion of Fig. 2. At the end of the time interval regulated by these relays, if an idle sender has not been selected, the link control circuit is released and disassociated from the trunk actuating circuit in the following manner: When the link control circuit was seized by a trunk circuit, relay 2 i was operated which energized relay 23s as previously stated. This associates ground from contact l of relay 2M with the movable member 3 of interrupter 225 which alternately closes its contacts l and 2 in measured time intervals regulated by the interrupter cam 4. The first closure of contact l energizes relay 2&6 from ground, through contact I of relay 2M, contact i of interrupter 225, contact 5 of relay 2E8, contact lof relay 2:9, winding of relay 235 to battery. Relay 2H3 locks over a circuit from battery through its winding and its contact 2, contact I of relay 2H to ground on contact 3 of relay 2H4. When interrupter 225 associates its member 3 with contact 2, ground is connected from contact I of relay 2% through contact 2 of the interrupter, contact 3 of relay Eifi, contact 4 of relay 258, winding of relay 2553 to battery operating relay 2L9. This relay locks through its contact 2 to ground on contact 2 of relay 264. The second association of the member 3 with its contact l energizes relay 2!! from ground on contact I of relay 2H5, contact 5 of interrupter 225, contact 5 of relay 218, contact I of relay 2H9, winding of relay 2!? to battery. Relay 25? is locked through its contact l to ground on contact 3 of relay lid. The operation of relay 2H releases relay 2H5 since relay 215 was held operated through contact I of relay Zll and since the original operating circuit for relay Zia: was carried through contact of relay 2l9. The second association of member 3 of interrupter 225 with contact 2 energizes relay 2!!! from ground on contact I of relay 2M, contact 2 of interrupter 225, contact 3 of relay 215, contact 2 of relay 2H winding of relay 258 to battery. This completes a time interval in which the testing facilities of the control circuit may be associated with a number of groups of senders in an eifort to find an idle sender for completing a telephone connection. If no idle sender has been found during this interval, the control circuit selected is now disassociated from the trunk but the link switching system remains associated with this trunk. This condition is recognized by the timing circuit devices when the interrupter again associated its member 3 with its contact l. The release of the control circuit is made when a group of senders is being tested and no idle sender is found in this group. Consequently, relay 2% is operated and relays E39 and Mt are normal. Under the several conditions outlined above a circuit is again established for relay 2E6 from ground through contact I of relay 2M, contact I of the interrupter, contact 5 of relay 218 which was thhe last timing relay to be operated, winding of relay 25-3 to battery. Relay 2H5 now closes a locking circuit for itself through its contact 2, contact 5 of r lay 2H3, to ground on contact 3 of relay 25 .1 and establishes an operating circuit for relay 252. This circuit may be traced from battery, through the winding of relay 282, contact 2 of relay 2i 1, contact I of relay 21B, contact 2 of relay 2l8, contact 2 of relay 53.5, contact 4 of relay 26%, contact 2 of relay 14$, con" tacts i or 2 of relay 2&5 to ground either on contact 3 of relay or contact 3 of relay 2 3i. It will be noted that the foregoing conditions for establishing the circuit for relay H2, 1. e., with relay 2&5 operated and relays I39 and Mt normal is the same condition which prevails for energizing relay 2. It will also be noted that the circuit for relay 2l2 was carried through contact 2 of relay 2% l. Relay 2!! is slow-to-operate and consequently a circuit is established for relay 2!! after the second operation of relay 256 before the slow-to-cperate relay 2i! time to operate.

Rclcase of link control circuit The operation of relay 232 associates ground with control circuit release relay 222 operating relay 222 which opens the circuits for relays M5 and 2M causing their release. The release of relay 2M restores the timing relays Bit, 2H, H8 and 2E9 to normal. Relay 222 remains locked through ground on contact l of relay E28 and does not release until the link control equipment has all been released. The release of relay 255 disassociates ground from the operating circuit for trunk group relay I29 and also disassociates ground from the switching relay 2H3 to prevent further functioning of this relay combination. The release of relay opens the holding circuit for the start relay l2? and opens the circuit for the slow-to-release relay 53. The release of the start circuit 32? opens th circuit for the sequence relays in the connector E29 which releases and opens the circuit for the multicontact relay I28. The equipment of the link control circuit is thus entirely released and the connector and trunk start circuit lead is momem tarily disassociated from the link switching sys tem by the continued operation of the slow-torelease relay 2%. After a short time interval, relay I59 releases again establishing the start circuit from the trunk circuit ltd. If at this time the trunk circuit still requires the services of a link control circuit for obtaining an idle sender the start circuit continues to be energized in the trunk circuit and again causes the operation of the start relay til. This energizes the sequence circuit in connector 529 causing the selection of another link control circuit. The second link control circuit taken for use functions in the same manner as the link control circuit herein described but has access. to different senders in the groups of senders associated with this second control circuit and an idle sender is ordinarily found by this second control circuit even during the heaviest telephone call load periods and, consequently, with this arrangement an idle sender is usually found in a period not to exceed ten to fifteen seconds.

For the following description, the trunk circuit Hit may be assumed as either a two-way trunk circuit combinin the functions of an incoming and an outgoing trunk or a one-way outgoing trunk circuit. In either case the outgoing trunk requiring the services of an outgoing sender energizes a start circuit H32 by the energization of a relay in the trunk circuit which associates ground through the contact of relay I50, winding of start relay I21 to battery operating the link start relay. This outgoing trunk also associates ground through resistance IOI for transmitting a signal to a link control circuit after it has been selected. The energization of the start relay I21 selects a link control circuit in the manner previously described and energizes relay I28 for associating the trunk circuit with the selected link control circuit and for associating the control circuit testing facilities with the contacts of the link frame relays I23 and I25. The off-normal relay 2I5 is energized from ground on contact I of relay I28 and a trunk group relay such as 226 is energized from ground on contact I of the start relay. As previously stated this start relay is associated with a particular group of trunks and may or may not be the same start relay used for the selection of incoming senders. The trunk group relay 226 will probably be a different trunk group relay than the one used for the selection of incoming senders. After the operation of a trunk group relay such as 226 and the off-normal relay 2I5, a circuit is established for relay I26 which connects the windings of the link frame relays with the control apparatus of the link control circuit. At this time the class relay is operated by the trunk signal for indicating to the link control circuit the character of equipment such as outgoing senders to be selected. Relay 202 has been shown for this purpose and a circuit is established from battery through its winding, conductor 231, through connector I29, contact I3 of relay I28, trunk resistance IM to ground. Relay 2 I 5 connects ground from its contact 3 through contact I of relay 2, winding of relay 2I0 to battery for energizing relay 2I0. Assuming that relay 201 is normal, the energiza- .tion of relay 2I0 causes the operation of relay 209 in the manner previously described. A circuit for relay 200 is established from battery through resistance 208, contact I of relay 201, winding of relay 200, contact 3 of relay 2 I0, contact I of relay 205, contact 3 of relay 20I to ground. This establishes a circuit for associating the link control circuit testing facilities with the first group of outgoing senders by energizing relay I23. The circuit for energizing the latter relay may be traced from ground through the winding of relay I23, contact 3 of relay I26, contact 4 of relay I28, through connector I29, conductor 233, contact 2 of relay 200, contact I of relay 202 operated, contact I of relay 203 normal, contact I of relay 204, resistance 228 to battery. The operation of relay I23 associates the senders of outgoing sender group III through an equivalent number of conductors II6 to H1, an equivalent number of contacts such as I and 2 of relay I23, thence over conductors I62 and I63, contacts 9 and I0 of relay I28, through connector I29, contact 2 of relay I3I, winding of relay I35 to battery and contact 2 of relay I30, winding of relay I34 to battery. The number of relays I34 and I35 is equivalent to the number of senders in group III. The operation of relays such as I34 and I35 when an idle outgoing sender is connected thereto is the same as described for incoming sender testing facility relays I36 and I31. A ground in an idle outgoing sender indicates that the sender is idle and operates the relays I34I35 corresponding to the idle senders. The operation of any one of these relays establishes an obvious circuit for relay I39 to prevent the link control circuit from making further search for an idle sender. The operated relay also energizes relay I4I which functions as before and energizes relay I38 for associating ground from an energized relay I34--I35 whichever is the first in the sequence of relays energized. For example, if relay I35 is associated with an idle sender, ground is connected through the contacts I of relays I36 to I31 which are normal, contact I of relay I35, contact 2 of relay I38, through connector I29, winding of link switch magnet I01 to battery which energizes this holdin magnet the select magnet associated with a particular horizontal level having previously been operated.

When all outgoing senders in the sender group I I I are found busy, the link control circuit automatically progresses for associating the control circuit testing facilities with another group of outgoing senders. When relay I23 was operated for associating the testing facilities of the control circuit with the sender group III, a circuit for relay 205 was established in the manner previously traced. The operation of relay 205 establishes an energizing circuit for relay 2 from battery, through its winding, contact 2 of relay 204, contact 3 of relay 205, contact I of relay I40, and contact I of relay I39 to ground. It is apparent that relay I39 remains normal if no sender in the group is found idle. Relay 2 opens the operating circuit for relay 2I0 in the manner previously described which causes the operation of relay 201 by removal of a shunt around its winding. This shunt may be traced from the contact of relay 2I0 when operated, contact 3 of relay 201 normal to a point between resistance 250 and the winding of relay 201. Since relay 209 is operated, ground is associated through its contact with the upper terminal of the winding of relay 201. The release of relay 2I0 consequently establishes an operating circuit for relay 201 from ground through the continuity contact of relay 209, winding of relay 201, resistance 250 to battery. The operation of relay 201 opens the circuit for relay 200 which releases relay I23 and the releaseof relay I23 opens the circuit for relay 205 causing the release of relay 2. The release of relay 2 reenergizes relay 2"! and a circuit is established for relay 20I. The circuit of relay 20I may be traced from battery through the resistance 208, contact 2 of relay 201, Winding of relay 20I, contact 2 of relay 2I0, contact 2 of relay 205 to ground on contact 3 of relay 200. The reoperation of relay 2I0 also places a shunt around the winding of relay 209 to cause its release in the manner previously described. Relay 201 remains operated to ground on contact I of relay 2I0. The operation of relay 20I now establishes a circuit for link frame relay I25 to associate the testing facilities of the link control circuit with the senders of sender group II3. A circuit for relay I25 may be traced from ground, through its winding contact I of relay I26, contact 2 of relay I 28, through connector I29, conductor 23I, contact 2 of relay 20I, contact I of relay 202, contact I of relay 203, contact I of relay 204, resistance 228 to battery. The senders of outgoing sender group II3 are connected to the testing facility relays over conductors I20 and I2I, contacts I and 2 of relay I25 which are equivalent to the number of senders in the sender group, thence over conductors I62 and I63, contacts 9 and I0 of relay I28, through connector I29, contact 2 of relay I3I, winding of relay I35 to battery, contact 2 of relay I30, winding of relay I34 to battery. If an idle sender is found in the second group, the link control cir-' cuit equipment is opened so that the sender is taken for use. If all of the senders in the second group tested are found busy, the control circuit equipment continues to transfer the control circuit testing facilities from one group to another until disconnected by the timing circuit relays as previously described. Also, as previously described, after the control circuit is disconnected from the outgoing trunk and the trunk continues to energize its link start circuit another control circuit may be taken for use to find an idle outgoing sender for completing the telephone call.

When an idle sender is found in the first group tested or in subsequent groups tested the testing circuit 206 is associated with the idle sender taken for use. If this testing circuit is satisfied the test relay is operated for energizing relay 204 which opens the control circuit equipment through its contacts as shown, places a ground on conductor 252 for maintaining relays 201 or 209 in the position taken and also connects a ground through the winding of relay 252 to battery for operating the latter relay. This estabstart relay I21 will not be again operated as it is when all of the senders tested are found busy.

The timing circuit relays 2H5, 2, H8 and 2l9 are associated with a trouble alarm circuit if under certain conditions a fault exists in a link control circuit which prevents it from operating satisfactorily. The timing circuit relays operate in the same manner as previously described and it will be remembered that relay M5 was operated and then released when relay 218 was operated. Under the condition for releasing the link control circuit previously described, a circuit is established for relay 2i 2 through contact I of relay 2l6, contact 2 of relay H3 and through other contacts of the link control circuit which must be in a particular position for the circuit to be established. When this release circuit is not established the interrupter 225 continues to function and when its member 3 reaches its contact 2 a circuit is established from ground through contact I of relay 2M, member 3 and contact 2 of interrupter 225, contact 3 of relay 2165, contact 4 of relay 2l8, winding of trouble relay 220 to battery. The energization of this trouble relay associates ground through its contact to a trouble alarm circuit which lights a lamp to indicate the link control circuit in trouble and also sounds an alarm which may be accomplished in any manner shown in trouble alarm circuits known in the art.

What is claimed is:

1. In a telephone system, trunks, a link switching system, a plurality of groups of trunk connecting devices available to said link switching system, each group comprising a plurality of trunk connecting devices, means in a trunk for actuating said link switching system, testing facilities in said link switching system arranged to simultaneously test the trunk connecting devices of a group, an automatically operable switching device for successively and repeatedly switching said test facilities from one group of trunk connecting devices to another for successively and repeatedly testing the trunk connecting devices of said groups and for selecting an idle trunk connecting device in one of said groups, and means in said testing facilities for arresting the action of said switching device when an idle trunk connecting device is selected.

2. In a telephone system, trunks, a link switching system, a plurality of groups of trunk connecting devices available to said link switching system, each group comprising a plurality of trunk connecting devices, means in a trunk for actuating said link switching system, testing facilities in said link switching system arranged to simultaneously test the trunk connecting device of a group, an automatically operable switching device for successively and repeatedly switching said test facilities from one group of trunk connecting devices to another for successively and repeatedly testing the trunk connecting devices of said groups and for selecting an idle trunk connecting device in one of said groups, means in said testing facilities for arresting the action of said switching device when an idle trunk connecting device is selected, and means in said link switching system for connecting the selected trunk connecting device to said trunk.

3. In a telephone system, trunks, a link switching system, a plurality of groups of trunk connecting devices available to said link switching system, each group comprising a plurality of trunk connecting devices, means in a trunk for actuating said link switching system, testing facilities in said link switching system arranged to simultaneously test the trunk connecting devices of a group, a timing mechanism, an automatically operable switching device for successively and repeatedly switching said testing facilities from one group of trunk connecting devices to another for a time period regulated by said timing mechanism for successively and repeatedly testing the trunkconnecting devices of said groups and for selecting an idle trunk connecting device in one of said groups, and means in said testing facilities for arresting the action of said switching device and said timing mechanism when an idle trunk connecting device is selected.

4. In a telephone system, trunk circuits, a link switchingsystem including a plurality of switches and a plurality of selectable link control circuits, a plurality of senders in groups available to each of said plurality of link control circuits, means in a trunk circuit for actuating a link switching system causing the selection of an idle link control circuit, testing facilities in said control circuit for simultaneously testing the senders in a sender group, an automatically operable switching device in said control circuit for successively and repeatedly switching said testing facilities from one group of senders to another for successively and repeatedly testing the senders of said groups and for selecting an idle sender in one of said groups, and means in said testing facilities for arresting the action of said switching device when an idle sender is selected and for operating a link switch to connect the selected sender to said trunk circuit.

5. In a telephone system, trunk circuits, a link switching system including a plurality of switches and a plurality of selectable link control circuits, a plurality of senders in groups available to each of said plurality of link control circuits, means in a trunk circuit for actuating a link switching system causing the selection of an idle link control circuit, testing facilities in said control circuit for simultaneously testing the senders in a sender group, an automatically operable switching device in said control circuitfor successively and repeatedly switching said testing facilities from one group of senders to another for Suecessively and repeatedly testing the senders of said groups and for selecting an idle sender in one of said groups, means in'said testing facilities for arresting the action of said switching device when an idle sender is selected and for operating a link switch to connect the selected sender to said trunk circuit, and a timing mechanism for arresting the action of said switching device when an idle sender is not selected within a predetermined time period and for disconnecting said control circuit from said trunk circuit.

6. In a telephone system, a trunk circuit, a

link switching system including a plurality of switches and a plurality of selectable link control circuits, a plurality of senders in groups with a plurality of different groups available to each of said plurality of link control circuits, means in a trunk circuit for actuating a link switching system causing the selection of an idle link 'control circuit, testing facilities in said control circuit for simultaneously testing the senders in a sender group, an automatically operable switching device in said control circuit for successively and repeatedly switching said testing facilities from one group of senders to another for successively and repeatedly testing the senders of said groups and for selecting an idle sender in one of said groups, means in said testing facilities for arresting the action of said switching device when an idle sender is selected, a control circuit disconnecting relay, a timing mechanism ,for operating said relay to disconnect said link control circuit from said trunk circuit when no idle sender is selected within a predetermined time period and means in said link switching system responsive to the operation of said disconnecting relay for causing the selection of another link control circuit for successively and repeatedly searching other groups of senders to select an idle sender for connection to said trunk circuit.

'7. In a telephone system, trunk circuits, a link switching'system including a plurality of switches and a plurality of selectable link control circuits, a plurality of diiferent character of senders in groups,- like characters of senders being in the same group with a plurality of groups of each character of senders available for connection to any of said plurality of link control circuits for selecting idle senders for trunk connections, means in a trunk circuit for actuating a link switching system for causing the selection of an idle link control circuit, selective means in said control circuit for determining what character of senders shall be connected tc said control circuit for selection, means in said trunk circuit for operating said character determining means, testing facilities in said control circuit for simultaneously testing the senders in a grou of the character so determined, an automatically operable switching device for successively and repeatedly switching said testing facilities from one group of senders to another of the same character for successively and repeatedly testing the senders of said groups and for selecting an idle sender in one of said groups, and means in said control circuit for arresting the action of said switching device when an idle sender is selected.

8. In a telephone system, trunks, a link switching system, a plurality of groups of trunk connecting devices available to said link switching system, each group comprising a plurality of trunk connecting devices, means in a trunk for actuating said link switching system, testing facilities in said link switching system arranged to simultaneously test the trunk connecting devices of a group, an automatically operable switching device for successively and repeatedly switching said testing facilities from one group to another for successively and repeatedly testing the trunk connecting devices of said groups and for selecting an idle trunk connecting device in one of said groups, a timing mechanism for controlling the duration of time of switching said testing facilities from one group of senders to another for finding an idle trunk connecting device, a link switching system release circuit and means in said switch device controlled by said timing mechanism for switching said testing facilities to energize said release circuit during the test of the trunk connecting device in the last group having no idle trunk connecting devices therein.

ALOYSIUS J. BUSCH. 

